The present invention relates to an optical disc and an apparatus for reproducing signal therefrom, and more particularly to an optical disc and an apparatus for effectively cancelling a crosstalk component mixed into a read-out signal of a target track from a plurality of neighboring tracks.
Recently a high density recording of an optical disc has been promoted or developed using various methods. For example, a method of enhancing the density of information pits in a direction along a track, a method of minimizing a diameter of spot of reading laser beam, and a method of reducing a track pitch are all known.
However, the method of reducing the track pitch has a restriction due to the diameter of an optical spot converged on a disc. Namely, if only the track pitch is reduced without reducing the diameter of spot of the laser beam, signals read out from neighboring tracks would be mixed into a read-out signal from a target track. Hence, a crosstalk component would be increased and it becomes impossible to accurately reproduce a recorded signal.
In order to cope with the adverse affect of the crosstalk phenomenon, the following countermeasures have been proposed. According to one method, a crosstalk component mixed into a read-out signal of a target track from inner and outer neighboring tracks thereof is detected and eliminated by reading the three tracks with three laser beams and subtracting the crosstalk component that has been generated based on two read-out signals of the neighboring tracks from the read-out track of the target track. Japanese Patent Application Laid-Open No. HEI 3-40225 discloses a method in which a frequency variable filter is provided for varying a frequency characteristic thereof so as to cancel a crosstalk component and the filter response becomes an optimum condition. In addition, there is proposed a method of deciding a cancellation coefficient of crosstalk component so as to minimize a correlation between the read-out signals of the neighboring tracks. This method is disclosed in "Nikkei Electronics" (Nikkei BP company, Vol. 495, Pages 98 to 99, Mar. 19, 1991).
FIG. 1 is a block diagram showing an apparatus for reproducing signal from an optical disc disclosed in the above-identified Japanese publication HEI 3-40225. The optical disc reproducing apparatus includes photo detectors 50, 51 and 52 for receiving reflected beams, respectively, from three tracks onto which light beams are protected and for photoelectrically converting the received beams into electric signals, frequency variable filters 56, 57 and 58, an adder 59 and a discriminator 60. The adder 59 adds the respective read-out signals of the three tracks after filter-processing by the frequency variable filters 56-58 so as to cancel the crosstalk component mixed into the signal of the target track. Filter coefficients of the frequency variable filters 56, 57 and 58 are renewed using a timing controller 53, a switch 54, a reference signal generator 55, a filter controller 61 and an error signal detector 62. More specifically, a predetermined preamble signal is recorded in advance on an optical disc and the preamble signal is initially reproduced prior to the reproduction of recorded data signal. The reproduced preamble signal which is outputted from the adder 59 and includes a crosstalk component and an interference is compared with a reference signal which is equal to a preamble signal recorded in advance, so as to detect an error therebetween. In response to the detected error, the respective frequency variable filters 56, 57 and 58 are controlled to renew their tap coefficients. As a result, a replica of the crosstalk component mixed from each neighboring tracks is outputted as reverse characteristics from each of the frequency variable filters 56, 57 and 58. Hence, the crosstalk component is removed in the adder 59 by adding the replicas outputted from the frequency variable filters 56, 57 and 58.
As described above, the conventional optical disc reproducing apparatus generates a signal corresponding to the crosstalk component from the neighboring tracks to thereby remove the crosstalk component. However, the above-described apparatus requires a circuit for generating the reference signal which is similar to the preamble signal beforehand recorded in the optical disc and a circuit for detecting the error between the preamble signal and the reference signal. This makes the circuitry complicated. Further, the above-described conventional method is only applicable to a digital signal processing.